2 different versions?

While it's impossible to install Forge for two versions on the same version simultaneously, let alone two different versions of Minecraft, you can, however, install different versions of Forge for these versions... but as icephantom said, they are vastly different and incompatible with each other if you try to use the same mods. If you do, the game will either refuse to launch or bug out horribly when you enter a world.

Also elaborating on what they said, you can use MultiMC to make two different instances of Minecraft... but due to the way Forge in 1.14 (or was it 1.13?) and future versions work, it can't actually download and patch it automatically, meaning you'll have to do it yourself. In that case, the vanilla launcher might be your best bet, and you can make different installations for each version, and you might want to have a different directory for Minecraft for one of your installations if you do so.

You can have multiple forge versions at the same time - in official launcher go to installations tab and create new installation - there select another forge version and in game directory area change .minecraft to anything you want

There is a version of forge for 1.12.2 and there is a version of forge for 1.14.4. They are different and not compatable with one another. However, if you use a launcher such as multimc, which you can find here : https://multimc.org/ then you can set up an instance for 1.12.2 with forge, and a seperate instance for 1.14.4 with forge.
I find it doubtful that you are trying to simultainiously start 2 versions of minecraft at the same time, so I assume you just want to be able to set up multiple, but seperate instances - which is what multimc excels at !