After playing on the website, I still have a couple questions.
My partner has a senior picture ID card we got last year from the downtown MTA office, so I know he can get a 30 day unlimited reduced fare card for $40.50. My 30 day unlimited card will cost $81.
But do we buy both from a vending machine? Or do we need to go to a serviced booth to show his ID card to get his senior one?
Also, do I remember that these passes are not good on Express busses?

You can use the vending machines. In fact in many stations, there are no longer any "token booth" attendants!

The senior/disabled reduced fare card is coded so that it will charge half price for fares--half price for unlimited, and if you just buy regular("pay per ride") amounts, it will only take off the half priced fare when you use it.

When you use the vending machine, you need to pick the option to "add onto an existing metrocard" (or however it is worded) so that when you put the card in the machine, it will charge the correct amount.

Correct that the "regular" passes do not include express busses, but I think you can still buy an "express bus included" pass for more money. Most of the express busses go out of Manhattan (e.g. Riverdale)--if you don't plan on using a lot of them you can always pay a la carte for those trips

I'm not sure about where to fill the senior fare card but I do know that you cannot use the 30 day unlimited ride on the express buses. Unfortunately it won't even let you swipe that card and then add the difference in fares. You would have to use a different card. Are you thinking of venturing into Brooklyn?

one addition/correction:
The only pass that includes the express busses is now the 7 day special pass at $41 (vs $25 without express bus privileges) and there is NO reduced fare discount on that card http://mta.info/metrocard/mcgtreng.htm#unlimited

OK, now I'm more confused than before about the senior card. As I recall that MTA ID card is something he carries with him, but he needs to purchase an unlimited card. I don't think his picture ID card gets loaded, does it? Last year, I know we bought his half fare unlimited metro card at the office when they printed and did the ID card, but it seems they were two separate cards, and we don't have the regular metro card, only the ID card.

That's what I was remembering about the express busses. But if we go to Brooklyn, we'd probably only be using the subways anyway. I just remembered the issue with the express bus when we were going to take the bus from midtown to the Bronx Zoo.

Hmmm. What all else fails -- look.

I just looked at his ID Card. Maybe I am remembering wrong (now, there's a surprise!). The front is mainly green, has his picture and a big R on it. On the back it does sort of look like a metro card with one cut corner and a magnetic strip. So we actually load that card? Why was I thinking that one stayed in his wallet and we had to get another card to swipe each time?

Yes, you load up that card.

The last time there was a separate card vs paying system was when we still used subway tokens.
If you were using a bus and paying cash, I suppose you could use that card as valid ID (for the half fare)without actually loading up the card, but for subways, it's only using the special card

I didn't think seniors could get a 30 day unlimited loaded onto their cards!

In any case, I checked the MTA's FAQ website (www.mta.info), and here's what I found:

How do I put fare on my Reduced-Fare MetroCard?
You can add value to your Reduced-Fare MetroCard in four ways: through Reduced Fare Mail & Ride or at MetroCard Vending Machines, MetroCard buses or vans, or subway station booths.

how do you get a reduced fare for a single trip?