No and yes, `emerge` like `pacman` discourages multiple instances. A lock is created during the dependency checking phase(the dependency graph is then put in RAM) and the install phase, but not during the compile phase, as @Eduardo noted, this allows multiple runs of `emerge`(and single writes to the `vdb`, but the same package can be compiled twice, which some consider counter productive. Most users choose to use the option `--keep-going` should breakage occur.
In order to speed up your `emerge --newuse --update` consider using the `--jobs` option. See Parallel Builds With Gentoo's Emerge, and have a look at `man emerge`