I do my very best to marry my batteries from the start. I think as others have said, well best practice is to start with two batteries, ordered together, and used together as well. However if cost is an issue, I do think you'd probably be fine adding another battery to the one you have, if it hasn't seen much use. Ideally, if you can get one from the same batch, well, that reduces one unknown variable.
However, provided you don't plan to use it in an unprotected mod, I'm inclined to think it would be fine. However, if you have a vape you are currently using the battery in (a single battery mod I guess) I might consider saving that battery for that mod, and just getting two new and paired from the start. This is, of course, assuming you want to continue using that mod.
Also, keep in mind, I am just a person on the internet, and best practice is to marry virgin batteries (hopefully one of them knows what they're doing, LOL) and keep them that way.
So ultimately, you will have to decide if this is a safe practice for you. I can only say that BEST practice is to marry from the start, and I tend to try and use best practices with batteries, they are are the single most important factor in vaping, IMO, as you really can't vape successfully if something blows up, I am not at all saying that yours won't do that if you used one a few cycles before marrying it, I find it somewhat unlikely, but I do generally tend to recommend best practices concerning batteries.
Best of luck deciding,
Anna