Scapeshift

[Vindog]

How does this work and how does it work with 2 valakuts out in play before the scapeshift

[Agrus Kos, Enforcer of Truth]

Here are a list of rulings from Gatherer (if it helps):

10/1/2009    Valakut, the Molten Pinnacle's triggered ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless, at the time a Mountain enters the battlefield under your control, you control five or more Mountains other than that new one, and (2) the ability will do nothing if you control fewer than five Mountains other than that new one by the time it resolves.
10/1/2009    If a Mountain you control leaves the battlefield between the time Valakut's second ability triggers and the time it resolves, be aware if that was the Mountain that caused Valakut's ability to trigger or not. If it was, Valakut's count isn't affected; if it wasn't, Valakut's count goes down by one.
10/1/2009    If multiple Mountains enter the battlefield under your control at the same time, Valakut's second ability could trigger that many times. Each ability takes into consideration the other Mountains that entered the battlefield at the same time as the one that caused it to trigger.
10/1/2009    You target a creature or player when the ability triggers. You decide whether to have Valakut deal damage to that creature or player as the ability resolves.

[Agrus Kos, Enforcer of Truth]

How does this work and how does it work with 2 valakuts out in play before the scapeshift

To more directly answer your question. When you {Scapeshift} all of the lands you get enter at the same time and "witness" each other entering. It's weird, but it works. So if you have 2 {Valakut, the Molten Pinnacle}s on the field and then you {Scapeshift}, as long as you get 5 or more mountains, both {Valakut, the Molten Pinnacle}s will trigger. So five mountains gives you 30 damage, 6 gives you 36, etc.

[Vindog]

How does this work and how does it work with 2 valakuts out in play before the scapeshift

To more directly answer your question. When you {Scapeshift} all of the lands you get enter at the same time and "witness" each other entering. It's weird, but it works. So if you have 2 {Valakut, the Molten Pinnacle}s on the field and then you {Scapeshift}, as long as you get 5 or more mountains, both {Valakut, the Molten Pinnacle}s will trigger. So five mountains gives you 30 damage, 6 gives you 36, etc.

So do the valakuts have to be there before the scapeshift or can they enter with the mountains for damage to happen?

[Remillo]

The Valakuts can enter alongside the Mountain and 'see' each of the other lands enter at the same time.  Just make sure you have the required number of mountains and you're set!