Making a spell like this that works for ANY two spells is very tricky. Keep reading.
If you want to make a spell to combine Invis and Clinging for example I think you could use a compound power with Trigger as a naked advantage on one of the two powers. So when the Merge Invis/Cling spell is cast that allows the other two to be fired off after the triggered power is cast. Here is the idea...
Spell #1 - Invis
Spell #2 - Clinging
Spell #3 - Merge #1 and #2 = Naked Trigger on Clinging
In practice the mage would cast Merge first. This adds Trigger to Clinging. He then casts Clinging to set the spell. This can be done well before entering that nasty dungeon. When he wants to get away he casts Invis which triggers Clinging.
Personally as a GM I would only allow this version as it can be carefully controlled.
This version would also get very strange with attack spells as trigger does not COMBINE the powers it only allows one to follow the other.

Combining attack spells like an EB and a flash is tricky and VERY VERY powerful.
To merge attack spells I would buy the merge as additional points to one of the powers. Again I'll use an example.
Spell #1 - 8d6 EB
Spell #2 - 4d6 Flash
Spell #3 - 4d6 Flash linked to the EB (This means an 80 point active spell.)
Doing this for ANY combination of anything would require a VPP. I can't think of any other way to be flexible enough...
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