You can have up to 10 separate web applications on a farm. How many you should have is really up to your business needs. If you need to have two separate URLs for two completely different aspects of your business, 2 separate web applications is a good way to go. It's also a good way to go if you need different access levels between the two configurations (i.e. one that is intranet only and can get away with NTLM and one that uses Forms Based Authorization on the internet).
All in all, the fewer you can get away with maintaining, the better. Eventually you'll have to upgrade your SP instance and there may well be a period of time when you're maintaining 2010 and 2013 (or 2013 and 2016) farms, etc. As always, of course, business needs trump everything else.