You would effectively be running two scans, the on-access and on-demand scans and they work independent of each other, but they would both use system resources.
On-access scanner, this the active protection, any time you browse the web, run a program or open a file, etc. then Avasts shields would scan the content.
On-demand scans, scan all associated files relevant to the on-demand scan you chose, whether they are running or not.
Personally I feel these on-demand scans are of little use as if they were active they would be scanned by the on-access scanners.
- With a resident (on-access) scanner the need for on-demand scans is much depreciated. For the most part dormant/inert files are being scanned, the other active files are going to be scanned by the resident shields when they are activated.