It would be possible to replace one of the 90/90/50 Y brances with a 90/90/60, then perhaps add a 60/50/50 Y piece to a short length of 60mm id ducting to split the 60 down to 2 x 50mm.
see below links
https://www.butlertechnik.com/item/Webasto/Duct-Y-Branch-90-x-60-x-90mm-9009263B-1320707A/59P
https://www.butlertechnik.com/item/Eberspacher/Ducting-Y-Branch-60-x-50-x-50/62Y
https://www.butlertechnik.com/item/Eberspacher/or-Webasto-Air-Ducting-60mm-CUT-TO-LENGTH-102114310000-398497-Meter-Lengths/5JL
There will likely be a few ways of approaching this without necessarily finding a good 'bodge' ;)