About the only way to get the coax lengths right is to make one a little shorter than calculated then look at return loss of the two cavity filters when adjusted for resonance. Then you lengthen the cable using adapters until you see the return loss get better and better then start to get worse. Pick the total length that gives the best overall return loss.
If you make the cable out of Teflon coax like RG-142 and put an SMA connector on one end, you can use SMA male to female "plug savers" which have the same Teflon dielectric as the coax. After finding the right amount of adapters for the lowest return loss you can then make a new cable of the total length with adapters.
At the factory where they make and tune duplexers they have bins full of cables in different lengths and swap cables around during tuning to choose the right length cables.
It's usually a 1/2 electrical wavelenght between cavities. If you use RG58 it's 86" @ 45MHz but you should instead use double shielded coax to avoid any bleedover between coaxes. It's an emitter and a receiver antenna inside the cavities that's needs to be included in the lenght calculation and coaxes might have a slightly different transmission delay than what's in the specs and needs to be considered. And usually the finetuning needs to be done at the site.
/Ubbe