Here's an example of a good synergy:
Ars Magica 2's Sigil of Nature's Bounty makes things grow faster but requires no steady supply of anything unlike Thaumcraft's Lamp of Growth, and it can be upgraded to have a range of 10 unlike the Lamp's fixed 5. On the other hand, AM2 Sigils can cut down stuff but AM2 has no way to collect things, unlike Thaumcraft where you can do it with golems. So it's much easier to create a self-sustaining automated farm of most things, using only magic mods, if you have both mods.
Ars Magica 2's Sigil of Nature's Bounty makes things grow faster but requires no steady supply of anything unlike Thaumcraft's Lamp of Growth, and it can be upgraded to have a range of 10 unlike the Lamp's fixed 5. On the other hand, AM2 Sigils can cut down stuff but AM2 has no way to collect things, unlike Thaumcraft where you can do it with golems. So it's much easier to create a self-sustaining automated farm of most things, using only magic mods, if you have both mods.