Is it safe/possible to run multiple vulnerabilities scanners on the same target simultaneously?
Yes, using dnmap, it is possible to split a workload.
If you read the URL here -- http://sourceforge.net/projects/dnmap/files/ -- the author gives examples of simultaneous scans that are ok and ones that should be avoided. The general opinion is that running more than one scanner is fine as long as the target ports and/or hosts vary. In other words, if a single target port and a single target host is targeted by each scanner, then the benefits of automation decrease.