Yes, there was an issue with the original not starting from the beginning for reasons unknown but corrected in the pt. 2 iso. Burn your ISO to a DVD disk, not a CD. If prompted to burn as a MOVIE or data, choose MOVIE.
As for combining two ISOs... an ISO is a disc image. You can't burn several of them onto 1 disc. There may be workarounds to accomplish this but they are beyond my level of expertise. You may have to settle for two disks.
If you want to again play with burning via Videopad I'd suggest installing version 7.01 which is available here. Retain your original install file. Scroll down to the Videopad versions. This succeeded here, and the ISO created a nice DVD movie disk that should be good on the PC or a standalone DVD player.
Another usage of a DVD disk is simply storage. You might create an mp4 with VP, then burn the result to a DVD as data. This creates an ISO that contains the mp4, which can be played with Windows Media Player or the like. An mp4 probably won't be playable on a standalone DVD player. And a CD's capacity may be too limited to hold all the data.
In line with that, here, for a short time, here is the mp4 with initial scene intact. So, lots of options.
Your best bet might be to load mp4 clips one and two into VP, then burn them to a DVD data disk. If need be, the mp4 parts can be extracted from the respective iso files. Windows 10 should do this automatically with a double-click on the iso, which opens a virtual DVD drive that can be accessed via the Windows Explorer file manager. The mp4 will show up in that drive.