No, you can't run them concurrently. There is a registry hack to revert 7b2 to 6. If you really want to run both (in case you're a webdev for example) use Microsoft Virtual Machine.

I believe Microsoft has IE 6 available on FTP for those in dire need... assuming they didn't get rid of that with the SPA monkeybusiness. Or get firefox over ftp, ftp.mozilla.org.