Most boas switch easily from live to dead* prey (*either fresh killed or frozen-thawed)- have you ever tried offering dead prey? You should, it's much safer & snakes are no less enthused about grabbing their meal, but sooner or later your snake WILL get bitten by a live rodent, & even IF she "wins" the battle, she may be disfigured & harmed by an infected bite (also may need expensive vet care)...all of which you can avoid.
I don't want to advise you exactly how much to feed a snake that I cannot see, as prey size is related to the snake's body size, & different people (both sellers & snake owners) don't always mean the same thing by "small" or "medium". The best way to choose prey size is to feed a rodent that is slightly less wide than the widest mid-body part of your (un-fed) snake, & no bigger. This applies to boas & most snakes.
Don't feed 2 rats, you're asking for a regurgitation. If you feed what you think is an undersized rodent, feed only one, but give the next meal maybe a little sooner. A young boa that's 2 years old & big enough to consume sm/med rats should basically be fed every 2 to 2 1/5 weeks, & skip the meal when she's in shed. So if you feed a small rat instead of medium, you might feed again in about 12 days...ok? It's not an "exact science" but it's far better to under-feed boas a little, than to over-feed them a little.
It's what many of us call "grown slowly" & it's healthier for the snake.
Another plus for using f/t rodents is that you can keep a little supply on hand in the freezer, in case your source runs short of the right size when it's time to feed again.
If you have any difficulty with changing your boa to dead prey (or if changing from mice to rats, the same technique works well) there's an easy way to do it: first feed a much smaller rodent of the type your snake is used to taking, then once that is swallowed & they are still pumped up for food, then offer a smaller than normal rodent of the new type. They nearly always chomp it right down...this gets them used to accepting the "new food". Important note: the sum of both smaller rodents should not be bigger than what your snake is used to consuming as one rodent. After you feed a snake this way a few times, most readily take the "new" food just by itself.
To answer your last question: it is preferable to only feed one prey item, unless you have to use the technique I just described in order to switch her over. If & when you feed 2 prey items, they should follow closely together, otherwise your snake is likely to regurgitate them both. (The first gets partially digested, & the second one interferes with that, needing more digestive enzymes than what is available, & up it all comes!
It can harm the snake, and it smells really gross...a "lose-lose" deal. Rats are fairly challenging to digest, so feed only one sm or med at a time.)
And you sound like a well-intentioned owner.