Can you explain what you mean by merge?

I dont think you mean physically, so you must mean get the benefits of both.This can be done already.

Alternatively you could just as easily join the other clan.

Merging SH benefits would only help if tthere were different researches done I suspect that most clans would opt for the same,most beneficial researches,therefore there would only be a duplication of efforts, so no overall  benefit.

Why stop at two merging?Why not every SH?

That would just open up more problems. The bigger clans would be mad bc they couldn't do it bc clans can only have so many ppl in it. 

You'd run into leaders merging sh whenever they want regardless of what their clan wants. 

Those who are already maxed or close to it would feel slighted bc they worked hard to get their sh where it is. 

Yeah having to abandon your sh to join a better clan, sucks, but it happens. Would you rather sit in a small or dying clan bc you used some rss to build it? Or move to another clan where you have a better chance of winning cvc and clan events, which gets you nice rewards. 

You can also do what some alliances do. Leave a baby town in the old clan as the leader, go to the new clan for a couple cvc. That way the smaller clan gets a bit of a foot up with rewards. The other clan gets some help with the cvc and other clan things. Then at the end of the time period, go back to the old clan or choose to stay on with the new one.