Yes, with some caveats.
The 2 SSL hosts will need different public IP addresses for maximum compatibility. There is name based SSL, but Apache doesn't support that.
There's nothing stopping you supporting SSL for all pages on the main site and simply using
SSLRequireSSL for the relevant
/login area to deny access without SSL. Then you can simply direct people to
https:// from the signup page and back to
http:// when leaving the login page.
If you want to get a little fancier you could use the re-write options to redirect people who connect using the "wrong" protocol. I do something like that with Lighttpd.
Yes. You need 2 certificates and then use the SSLRequireSSL option for the parts you want to protect with SSL. I'd suggest that using a rewrite is kinder on visitors than simply denying access to HTTP connections to /signup in particular. See this page for an example of how to do that.