Award tickets can price higher as one way because the system is mixing different inventory buckets and sometimes applies married segment logic so when you split it the higher bucket shows up. The jump you saw from 255000 to around 400000 is normal when the cheaper partner space drops or gets separated from the connection. A simple fix is book the higher value leg first from dads account at 255000 each then wait a bit and book the other leg from moms account at 85000 each sometimes space realigns after a short time. Another option is switch the connection timing by a few hours same route different flight numbers often pull a lower bucket like 255000 instead of 384000. Also try holding seats if possible then ticket from both accounts quickly so the system does not reprice. Transferring miles is usually not worth it since fees can equal a cash ticket so better to work around availability and timing instead.