That could be a little tricky without a compound miter saw. I'm assuming the crown is too tall to cut in the vertical position?

The only thing I can think of is to make some kind of auxiliary table to support the moulding at the appropriate angle.

Make pencil marks on the fence and table so you can position both pieces the same.
Even better, you can add an insert table with a wooden stop strip that holds the molding at the same spring angle every time.
You can correct the second piece with a block plane as needed.
Casey

I have a compound miter saw but rarely use it in compound mode to cut compound miters. I much prefer to hold the molding upside down at the appropriate spring angle (pretend the fence is the wall and the table is the ceiling). It's just a lot more foolproof and when switching from left to right, you just swing to the right miter angle (e.g., 45 or 22.5) and ignore the tilt angle. So there's one simple setting instead of two complex ones every time you round a corner.

If you're doing a lot, make a little carriage that you can clamp to your table with a stop to hold the molding in the right position.

Scarf joints work better.