Ok (and how) to add a Mojave partition to Yosemite?

Here is an Apple article which explains how to download various macOS installers and to create bootable macOS USB installers:

https://support.apple.com/en-us/HT201372

Here is an article on how to erase a drive and install macOS. While it assumes installing to the internal drive, you can easily select an external drive as the destination when installing macOS (usually the external drive icon will appear as orange).

https://support.apple.com/en-us/HT208496

First make sure you have a good backup in case something goes wrong. Adding and deleting partitions is risky and could prevent the Mac from booting and even have you lose access to the data.

I personally don't recommend dual booting a system from the main internal drive. People almost always size the partitions wrong or discover that dual booting is a pain (it really is a pain) and end up needing to completely erase the drive and reinstall or restore from a backup.

How large is your SSD? I don't recommend dual booting to the internal drive unless you have at least a 500GB drive so both partitions have enough room. A 250GB drive divided in two just gives you about 120GB for each partition which is barely enough for each.

A better option is to install the second version of macOS to an external USB3 or Thunderbolt drive. It is best to use the internal drive for the more system intensive work and use the external for the lighter work.

No matter what always have frequent and regular backups for both systems.

Personally I would advise against creating a new partition for another macOS boot volume on the main drive since your drive is already full.

I don't do any editing so I'm not sure if a USB3.1 or 3.2 drive would be sufficient or not. If you have a spare USB3 drive you can certainly try it out.

What is the exact model of your laptop? You can get this information by clicking on the Apple menu and selecting "About this Mac". If you have a 750GB drive it sounds like you have a MBPro (mid 2012) non-Retina model using a hard drive. If this is the case, then you would be better off purchasing two 1TB SSDs and installing them internally by putting one into the optical drive bay (I recommend this be the Yosemite install). Besides replacing the original and very likely worn out hard drive it will give the laptop a nice performance boost plus give you slightly more storage space since you need to keep at least 20GB of space free on each boot drive. If you have the 13" model, then replacing the hard drive SATA Cable is highly recommended as well. I don't normally recommend using the optical drive bay for an SSD since it was never made to be used with a high speed SSD, but for editing it may be worth the risk over using an external boot drive. OWC sells the necessary hard drive SATA cable and an optical bay adapter for using an SSD. You can also use an OWC Mercury SSD or a Crucial MX500 series SSD.

I hope you do have frequent and regular backups of your system and any external media which contains important and unique data.

Editgrrl1 wrote:

It is a Retina model.

Is it possible to upgrade the graphics card in a laptop?

Is this something relatively easy to do? Or is it a "don't do this at home kids" repair?

Model Name: MacBook Pro

Model Identifier: MacBookPro10,1

The memory, CPU, and GPU are all an integrated part of the Logic Board and cannot be upgraded.

You also cannot upgrade the internal SSD if you want to use Yosemite on the internal SSD (or any OS older than 10.13). Your only option if you want to dual boot is to try installing macOS to an external drive (USB3 or Thunderbolt) to see how it works especially since your internal drive is already close to full.