I've tried to merge schematics in a simulator other than Altium. Here is the only method I am able to work:
1) open schematic #1 in the simulator,
2) save it all to clipboard,
3) Open schematic #2,
4) Paste schematic #1.
5) Both schematics appear side-by-side, unconnected.
Parameters need to be examined afterward, to check whether a circuit's parameters were kept.
Unsuccessful methods include:
* trying to merge netlists in word processor.
* open two simulators, copy and paste from one to the other.