For each of you:
o Two outfits (shirt, pants, undies, socks) -- one on you and one in your pack. Mix and match mean 4 different outfits.
o One wind shell - preferably with hood
o One insulation jacket (high fill-power down = max. warmth and min. weight/bulk)
o One umbrella
o COMFY walking shoes in solid colors (eg black or brown) good for long walks and presentable in the better restaurants
o Lightweight flip flops for beach and hotel wear (Old Navy sells them light and cheap)
Double check, but I don't believe Delhi is cold enough to warrant gloves or true winter gear
2 Toiletry kit - for 2 months, nothing bigger than 2-oz. sizes.
3. First Aid kit - Bear in mind India is VERY civilized and pharmacies stock EVERYTHING -- so just bring enough stuff to tie you over for 2-3 days -- and replenish locally. Exception - prescription drugs that you need - bring plenty of those.
4. Misc. stuff - pen, small pad, documents, debit and credit cards, emergency cash (US$), camera, batteries, AND Steripen Traveler or Adventurer.
All the above should fit very nicely in a 28L day pack. So you've got two day packs serving as your main packs -- and the two of you can use one for day time sightseeing.
I've traveled with my beloved Steripen all around the world in various trips over the last four years -- treating water in Bangladesh, Haiti, as well as 'cleaner' places. NEVER ONCE got sick. Saved a bundle on bottled water. Bring a wide-mouth Nalgene water bottle (20 or 24oz).
Hope this helps.