The ST length has no effect on the fit at all. All it does is change the amount of exposed seat post.

What is really important is the frame REACH. Reach takes into account the TT length and the effect of the seat tube angle. Two frames can have different TT length but fit the same, if the STA difference happens to cancel out the TT length difference.

Unfortunately, there is more than one definition for reach. You can get about the same information by comparing the TT lengths, with proper compensation for any difference in the STA. The compensation for STA is 8-10mm per degree, depending on the exact frame size. Steeper STAs make the reach longer.

If your frame with the 56cm TT has a 1 degree steeper STA than the other frame it would have a nearly identical reach, compared to your other frame.