Note: Simply alternating episodes won't quite work for this season (though it gets close).

Buffy -- The Freshman (401)
Angel -- City of (101)
Angel -- Lonely Heart (102)
Buffy -- Living Conditions (402)
Buffy -- The Harsh Light of Day (403)
Angel -- In the Dark (103)
Angel -- I fall to pieces (104)
Buffy -- Fear,Itself (404)
Buffy -- Beer Bad (405)
Buffy -- Wild at Heart (406)
Buffy -- The Initiative (407)
Angel -- RM W/A VU (105)
Angel -- Sense and Sensitivity (106)
Angel -- The Bachelor Party (107)
Buffy -- Pangs (408)
Angel -- I will Remember you (108)
Buffy -- Something Blue (409)
Buffy -- Hush (410)
Buffy -- Doomed (411)
Angel -- Hero (109)
Angel -- Parting Gifts (110)
Angel -- Somnamulist (111)
Buffy -- A new Man (412)
Angel -- Expecting (112)
Angel -- She (113)
Buffy -- The I in Team (413)
Buffy -- Goodbye Iowa (414)
Angel -- I've Got you under my Skin (114)
Angel -- The Prodigal (115)
Buffy -- This year's Girl (415)
Buffy -- Who are you (416)
Buffy -- Superstar (417)
Angel -- The Ring (116)
Angel -- Eternity (117)
Buffy -- Where the Wild Things Are (418)
Buffy -- New Moon Rising (419)
Angel -- Five by Five (118)
Angel -- Sanctuary (119)
Buffy -- The Yoko Factor (420)
Buffy -- Primeval (421)
Buffy -- Restless (422)
Angel -- War Zone (120)
Angel -- Blind Date (121)
Angel -- To Shanshu in LA (122)

There is some tricky ordering, so be careful or you may get the storyline out of order.

Buffy -- Buffy vs. Dracula (501)
Buffy -- Real Me (502)
Buffy -- The Replacement (503)
Buffy -- Out of My Mind (504)
Angel -- Judgment (201)
Angel -- Are You Now or Have You Ever Been (202)
Angel -- First Impressions (203)
Angel -- Untouched (204)
Buffy -- No Place Like Home (505)
Buffy -- Family (506)
Angel -- Dear Boy (205)
Angel -- Guise Will Be Guise (206)
Buffy -- Fool For Love (507)
Angel -- Darla (207)
Buffy -- Shadow (508)
Angel -- The Shroud of Rahmon (208)
Buffy -- Listening to Fear (509)
Buffy -- Into the Woods (510)
Buffy -- Triangle (511)
Angel -- The Trial (209)
Angel -- Reunion (210)
Angel -- Redefinition (211)
Buffy -- Checkpoint (512)
Buffy -- Blood Ties (513)
Buffy -- Crush (514)
Buffy -- I Was Made to Love You (515)
Angel -- Blood Money (212)
Angel -- Happy Anniversary (213)
Angel -- The Thin Dead Line (214)
Angel -- Reprise (215)
Buffy -- The Body (516)
Buffy -- Forever (517)
Buffy -- Intervention (518)
Buffy -- Tough Love (519)
Buffy -- Spiral (520)
Buffy -- The Weight of the World (521)
Buffy -- The Gift (522)
Angel -- Epiphany (216)
Angel -- Disharmony (217)
Angel -- Dead End (218)
Angel -- Belonging (219)
Angel -- Over the Rainbow (220)
Angel -- Through the Looking Glass (221)
Angel -- There's No Place Like Plrtz Glrb (222)

Buffy Season 6 and Angel Season 3 have no direct crossovers, but you should watch the first disc of Buffy before starting Angel for proper ordering. Here is the complete list:

Buffy -- Bargaining (Part 1) (601)
Buffy -- Bargaining (Part 2)(602)
Buffy -- After Life (603)
Buffy -- Flooded (604)
Angel -- Heartthrob (301)
Angel -- That Vision Thing (303)
Angel -- That Old Gang of Mine (302)
Angel -- Carpe Noctem (304)
Buffy -- Life Serial (605)
Buffy -- All The Way (606)
Buffy -- Once More, With Feeling (607)
Buffy -- Tabula Rasa (608)
Angel -- Fredless (305)
Angel -- Billy (306)
Angel -- Offspring (307)
Angel -- Quickening (308)
Buffy -- Smashed (609)
Buffy -- Wrecked (610)
Buffy -- Gone (611)
Angel -- Lullaby (309)
Angel -- Dad (310)
Angel -- Birthday (311)
Buffy -- Doublemeat Palace (612)
Buffy -- Dead Things (613)
Buffy -- Older and Far Away (614)
Buffy -- As You Were (615)
Angel -- Provider (312)
Angel -- Waiting in the Wings (313)
Angel -- Couplet (314)
Angel -- Loyalty (315)
Buffy -- Hell's Bells (616)
Buffy -- Normal Again (617)
Buffy -- Entropy (618)
Buffy -- Seeing Red (619)
Angel -- Sleep Tight (316)
Angel -- Forgiving (317)
Angel -- Double or Nothing (318)
Angel -- The Price (319)
Buffy -- Villains (620)
Buffy -- Two to Go (621)
Buffy -- Grave (622)
Angel -- A New World (320)
Angel -- Benediction (321)
Angel -- Tomorrow (322)

This is the last season when both shows were on the air. Unlike past episodes, some Angel episodes precede Buffy episodes, so the order is important for this season.

Buffy -- Lessons (701)
Buffy -- Beneath You (702)
Buffy -- Same Time, Same Place (703)
Buffy -- Help (704)
Angel -- Deep Down (401)
Angel -- Ground State (402)
Angel -- The House Always Wins (403)
Angel -- Slouching Toward Bethlehem (404)
Buffy -- Selfless (705)
Buffy -- Him (706)
Buffy -- Conversations with Dead People (707)
Buffy -- Sleeper (708)
Angel -- Supersymmetry (405)
Angel -- Spin the Bottle (406)
Angel -- Apocalypse, Nowish (407)
Angel -- Habeas Corpses (408)
Buffy -- Never Leave Me (709)
Buffy -- Bring On The Night (710)
Buffy -- Showtime (711)
Angel -- Long Day's Journey (409)
Angel -- Awakening (410)
Angel -- Soulless (411)
Buffy -- Potential (712)
Buffy -- The Killer In Me (713)
Buffy -- First Date (714)
Buffy -- Get It Done (715)
Angel -- Calvary (412)
Angel -- Salvage (413)
Angel -- Release (414)
Buffy -- Storyteller (716)
Buffy -- Lies My Parents Told Me (717)
Angel -- Orpheus (415)
Buffy -- Dirty Girls (718)
Buffy -- Empty Places (719)
Angel -- Players (416)
Angel -- Inside Out (417)
Angel -- Shiny Happy People (418)
Angel -- The Magic Bullet (419)
Angel -- Sacrifice (420)
Angel -- Peace Out (421)
Angel -- Home (422)
Buffy -- Touched (720)
Buffy -- End of Days (721)
Buffy -- Chosen (722)