Can I stack new beam to the top of old beam to extend span?
There is no problem flush framing the LVL in the ceiling as you suggest, provided it physically fits in the space available. If the LVL is placed directly over the 4x10, the ceiling joists will need to be cut to make room for the LVL. If this cutting keeps them from bearing on the 4x10, they need to be shored up until the LVL and hangers can be fitted. A temporary ledger on the 4x10 will suffice. Since LVLs can be narrower than sawn timbers, shoring/ledger may not be needed.

You might consider removing the 4x10 beam to open up the new opening so it is taller. Once you remove the supporting walls, it will more or less fall down anyway, unless it is well attached to the new LVL, essentially hanging it beneath the LVL. Why leave it there? It's not doing anything but getting in the way. Unless it's hiding the end of a soffited area, the taller opening would be an added benefit of flush framing the new LVL.