Is it a threat if top mining player merges? It just strikes to me that if top 2 bitcoin miners which would own more than 50% mining, merges can they control bitcoin.

In hypothetical scenario if have bad intension can they control the price etx of bitcoin ?
You are speaking of 51% attack or Majority Attack

You can read more about it in here

If they join, yes they can double spend or so.

But they can't change the price though, as that purely depends on the market demand.

Till now, there has been no Majority Attack on BTC, though it has been demonstrated on small altcoins.