Tunes for 10 Fingers and MFPA are like chalk and cheese - entirely different approaches. TFTF keeps the thumbs sharing middle C until well into the second book, so unless the teacher has used plenty of supplementary material( rarely, in my experience), transfer students often arrive never having read in a different position, typically for 12-18 months, using only 9 notes.
MFPA on the other hand starts with black keys and with the LH using different positions, so that the student reads the notes rather than associating a key with a finger. MFPA uses landmarks for bass clef reading with different fingers, moving to Bass C during the 2nd book. Intervallic reading is used rather than the individual letter approach in TFTF. Also MFPA has attractive duet accompaniments and CD backing tracks, while TFTF didn't ( the last time I looked) as well as complementary Writing Books ( not all that much writing, but plenty of excellent musicianship and technical activities).
MFPA is far from perfect, and Piano Adventures even less so (although I still use it for supplemantary work) but it's a good starter book for ages 4-6.
I currently have 2 siblings aged 18 months apart. I started them off at the same time using different materials ( Irina Mints Hello Piano! ) but then younger one moved into MFPA B and was romping through it, partly because he was playing mainly by ear, having heard his elder brother. The elder one is now using MFPA C along with supplementary books ( Little Monster Vol 2; Jane Sebba Piano Magic) and the younger is using a mix of Irina Mints Vol2, Denes Agay, and Hal Leonard. The boys are now aged 6 and 7 and have 45 minute lessons with Mum sitting in. They don't compare their progress with each other, but if and when they decide to take an exam I will probably let the elder take Grade 1 first.
You could either let your elder student move into an alternative series when he finishes MFPA B, in which case the younger one will probably play most of his tunes at least partly by ear, or let the elder move into MFPA C and find an attractive alternative for the younger (preferably not TFTF for reasons above ).