I think it is better to keep both sites on the VPS. It would be better if it is a managed VPS so your web host keeps an eye on it 24/7 as well.
Also make sure your VPS are backed up regularly.
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It would be best to keep both sites on the vps in the long run you'll have money. But the only think you have to worry about is having your eggs on one basket for example if the vps goes offline then both sites will be offline resulting in a loss loss situation. If you have them both on different servers then if one goes down you'll still have your other site that didn't go down online being able to shout news about the site that is down which helps because then it means you'll keep all your clients in the bubble with latest information about the other site that went down.
There are many other pros and cons as to why not to and why you should host the sites together or separate. It's up to you. But no matter what take into mind if you host both sites on vps just keep security tight. And always be sure to run update command and to always make sure all scripts and plugins are updated to latest update if not then prepare to be hacked.
I think you should be able to keep both on the same vps server but make sure you get the security checked out which will be very important. Make sure you have all your stuff on there up to date another important thing. You can use platinumservermanagement to check and make sure the security on your server is good to go. That's what I do