1. Are there any issues with linking to two separate CSS external sheets?
Link as many as you like within reason (so the performance of the page is not noticably affected).
Some designers link a dozen stylesheets and multiple Server Side Includes too.
2. Do you just put the link to both sheets one after the other?
Yes, just bear the rules for conflicting CSS rules in mind.
If a rule is similar or identical in both stylesheets (same specificity) and causes a clash then the rule in the second linked stylesheet (being the last declared rule) wins.
Also a rule in Stylesheet A may inadvertently cascade down to affect page elements controlled by Stylesheet B. These issues can be diagnosed with Dreamweaver's CSS editor or browser tools such as Firebug (for Firefox) and Developer Tools in IE.