I’m fitting an MT75 box in my Scimitar as stock the Scim propshaft is 1 piece, and seems quite long.

The MT75 will result in the prop being a bit longer, and also needing a sliding joint.

I’m worried the extra length and weight will cause whip and vibrations, bearing and seal wear.

Would a 2 piece prop be advisable?

Also how does the centre bearing on a 2 piece prop allow for gearbox movement?

I’m thinking of using mk5 Transit prop parts, as these were fitted with MT75s with the same output bearing but a plain shaft and no dampner.

I have limited experience of 2 piece props, but can offer an opinion.
the centre bearing is supported in a ring of steel, that bolts to the body/chassis. Between this ring and the actual bearing is a diaphragm of rubber. Always seems thin and insubstantial, but the damaged one on my old Triumph has done 15K in that state, but I am actually going to change it now it is all apart. As an aside, somebody has fitted a transit centre bearing to the same Triumph propshafts!

I think there are some concerns about angles, maybe as with 3 UJ's there will be a phase difference. I think, just think, the front section is supposed to be level? but do some research. Also remember that on the rear section, no UJ should have more than 3degrees of angle. I had a serious issue as I never considered that when I built my Zetec spitfire. Short prop meant angle was about 6 degrees, now down to 3.2 (as good as I can measure)

I expect there are some good guides out there on the WWW?

I would measure from the rear housing on the gearbox to the diff flange and have a word with a reputable prop shaft manufacturer for advice, also if you decide to go two piece make sure you have a suitable mounting point for the center bearing. There are less components on a one piece prop and it’s likely to be cheaper as well as more reliable? Can recommend Bailey Morris in Eaton Socon who I’ve found to be very helpful over the years , perhaps not the cheapest but they have a very good reputation.