Take webcam picture and screenshot at the same time
Something to try, add an ampersand to the end of your commands.
imagesnap -t 300&amp;

The idea here is that the ampersand will cause the command to run in the background allowing the loop to continue to the next step.
As an example, if you were to run the following in Terminal you'll see that the second sleep reports out before the first one.
sleep 10&amp;; sleep 5&amp;;

I hope this helps