it is definitely possible to merge 2 presets (or more) in a single one. There are some important aspects to take into account though. "90% of the programs" is an excessive extimation and there is no way to exactly know this as there are quite a number of different DSP resources that can be exhausted by an algorithm, making impossible to load it as a single DSP preset.
The art of Vsig programming is very deep and extremely rewarding as it unwraps levels of power and flexibility unknown to average fx usage…so taking the time to learn it will make your experience unextimable.
You can merge 2 presets into a single DSP or even into a monolithic one (the 2 DSPs work as a single one to load big structures that can't be run on a single chip). This is an automated process the Eventide will do for you at loading time.
With time and experience you will learn that there are parts of 2 presets you may cut out and/or change to a simpler form to help loading.
In general the answer is "YES, you can!"
all the best