Finding a merger partners is tough work as you have to be sure, as other have said, it's a good fit. That means, of course, not only goals, but playing styles, and a balance of boosted goods. And then you have to convince your players to move to the new fs.
Then there's the merger. Say you have 10 active players and they have 10 active players. Each side discusses it with their members -- but wait, of course only 3-6 actually discuss it and the others are pretty much silent...sigh. Both sides agree, it's a good idea....now who will be the AM? You can only have one. Depending on the personalities, that can tank the merger as well.
And, of course, should you start a new fellowship with both sides joining, one side invite the other into their fellowship? What about the "silent ones" in the fellowship into which you invite the other party? Were they actually in favor of the merger or not? And if not, in fairness to the "silent ones" in the other party who will be left behind (assuming they really don't want to leave), should you kick the "silent ones" in your fellowship? Or do you wait to see if they are actually in favor or not?
And on moving day, what if players you thought were moving just ignore it all? You'd be surprised how often that happens. Weeks go by and they remain in the old fellowship. Then they end up ignoring you. (I've seen this happen).
In the end mergers seldom work without the loss of about 1/3 to 2/3 of the members. But, the good news is, you have just breathed some life back into the two groups as they have made a statement about what they want -- an active and strong fellowship. So you end up with 13 players out of 20. Now all you need do is start recruiting. New blood will help the excitement of having done "something!" about the slowly dying fellowship.
So, in my opinion, it's a two step process. Merger and then recruit. Hard work but in the end probably worth the effort if you want your fellowship to really be strong.
AJ