How to install two different versions of Minecraft Forge?

Solution 1:

To answer your question: No, you can't have two versions of Forge installed over the same Minecraft installation, and neither can you have a mod compiled for a future/previous version of Forge run alongside those that has been compiled for said version.

So why?

Forge in itself, is a modification to the entire coding of Minecraft, allowing players to inject new code (hence mods) easily.
But because the structure of Minecraft's internal settings change from update to update, many methods get depreciated or moved (means things get messy).

That's why mods compiled for a release of Minecraft cannot run on another one.

So, what's the solution?

To simply put it, downgrade. Until there is an update to the mods (in this case, OreSpawn), that is your only option.

Seeing that you're trying to use Orespawn and Lucky Blocks, and that both mods have a 1.7.10 version, your best option is to downgrade back to 1.7.10 and install their respective versions of the mod for it.

As for missing features, you simply have to put up with it, since it's not possible to have a newer version of a mod run on an older version of Minecraft.

Minecraft Forge for 1.7.10 - Minecraft Forge
Orespawn for 1.7.10 - Original Website | Direct Download Link
Lucky Blocks Mod for 1.7.10 - Original Website | Direct Download Link

Note: When I say 'version/release of forge', I don't mean each individual build, but the targeted Minecraft version for them.

Seeing that you're proficient in Java on StackOverflow, you could (if the mods are open-sourced and you have knowledge in Minecraft modding) attempt to port the older version of the mod and make it (OreSpawn) compatible with 1.8's release(s) of Forge allowing you to utilize the newer version of Lucky Blocks and its new features (if any).