How to combine renders to form one seamless animation?

If I wanted to render say, frames 1 through 100 into standard quicktime format, then render frames 101 through 250 at a different time, would there be a simple way to somehow combine the two renders together to form one seamless animation?

I tried rendering the second set of frames to the same file source as the first set, but that just wrote over what was already there, and didn’t combine the frame sets together…

Render out as an image sequence (eg png in render output settings) setting start frame = 1 end = 100, then later do the same but setting start frame = 101, end frame = 250. Add them to the sequencer and output to whatever movie format you want.