we just went in october with our 4 year old and 2 year old twins. so while there were 2 of us, one of us always had 2 kids on a ride. at the mk I would say everything in fantasyland is doable for you and the 2 kids. same with toontown. you could do many other rides...pirates, the magic carpets, jungle cruise, haunted mansion (if you think your kids will be fine with it). as for AK, where the safari is, yes, they could both ride...there are several seats across. and no, you don't drive. (do you mean the ak safari here?) there is tons to do for that age group at all the park. my kids had a blast and talk about it still.
the hardest thing for me would be handling the double stroller/diaper bag, and kids on the bus, if you are using the bus system. the onloading/offloading would be tricky. but then you might find some kind soul who would help you get your stroller on/off while you get the kids on/off.
okay, just reread your post and realized you said
disneyland...do you mean disneyland (california) or disneyworld (florida)? I'm writing this from a florida perspective, and seeing you say safari makes me think that's what you mean. but either would be very doable for you.
to answer your other question, btw, no, your 3 year old will not be able to ride alone.