nice gloat on the 20" planer there jj ...

I ran mine through the planer in two passes (15" planer). You will
have no trouble lining up the two halves - no need to plane the top as
a unit. Just use lots of cauls to keep them aligned. Remember, you
can use the trestle as a caul. 3x3 maple doesn't flex regardless of
how much pressure you use. You're going to have to line up the aprons
any, and you can't run the whole top anyway. Good practice with a
handplane if it happens anyway. I had to plane the aprons to match the
top. Make sure the aprons are a tad higher rather than lower, the
latter is much more work to bring to level. Make sure the aprons can
slide on the end, I'd guess my bench moves a good 1/4" winter to summer.