Can your oldest go to preschool longer? That is only 2 hours...
usually, many preschools have half days... that would give you more time...
my daughter did half days, 3 times a week, when I had my 2nd child... and so my 2nd baby could nap, then when he woke, we'd go pick up my daughter, come back and he napped again. Because for the 1st year or so, my son was napping 3 times a day, as a baby.
Also, if your daughter goes to Preschool longer/half days... then you have more time with just baby and you. And your older daughter would then have more of a routine, for Preschool. Its hard to get in the groove for preschool, with only 2 hours... by the time they get into their Preschool routine/adjust, it is time for them to go home already etc.
If not, then yes, he will be napping in transit... and probably not getting a full uninterrupted nap.
My son, did NOT nap in transit... he was not 'portable' that way.
Neither was my daughter.
Some babies are more portable....
A 'routine' for naps, is ideal. Everyday.
If possible.