Amend your 2017 first. Then do your 2018 return with the new state refund from 2017. This is the minimum amount of paperwork.
If you can't amend your 2017 return before April 15th, then file an extension on your 2018 return before April 15th. Note that while filing an extension gives you up to 6 months to file, you don't have to wait 6 months - you can file the week after April 15th if you want.
Note that you don't have to wait for your amended return (which must be printed and mailed) to be accepted and processed by the IRS before you e-file your 2018 return.
Yes, since the 2017 amendment process at the IRS will take longer than the e-filing process for 2018, there will be a period of time in which there will be a mismatch on your two returns. Don't worry about it.
The initial testing on a return is usually on the year itself. Later the IRS will compare numbers between years, but that may be the middle of 2020, by which point your returns will be in sync.
And if in the unlikely case anyone ever writes to you about this, just tell them what you did. In fact, write down right now that you are choosing to amend your 2017 first, then file your 2018, because it didn't make sense to file the 2018, knowing you were just going to have to amend it. Stick this in your tax file.
