I have a wildcard SSL certificate setup on *.domain.com and its working fine. I now want to add a single ssl certificate to domain.com.
Is this likely to cause me problems? I assume not, but I thought it would be good to check that someone else has done it.
You need a different IP for each different SSL cert you use. So you'll need two IPs.
It's not clear why you want to use another SSL cert, is it an EV cert?
There shouldn't be a problem mixing the wildcard and specific certificates. Not sure why you would need to do this as the wildcard certificate should be able to handle all domain.com requests. You should also be able to bind the same wildcard certificate to multiple websites, meaning you don't need a unique certificate for each website.
If you have wildcard ssl certificate so why you want to purchase other ssl certificate.
Wildcard SSL certificate is the best solution for your requirement. It can secure your main domain (domain.com) and unlimited sub domains (*.domain.com). it means your main domain and all sub domains are secures with single wildcard certificate.