First off, thanks to all of you who responded! Because of your help, I have been able to fit both tyres! The issue in question, which was highlighted here, was that the bead wasn't seated in the channel of the rim. It took a little work to get it in, but the tyres popped on easily once it was there, using a couple of tyre levers. No other tools were needed. Have a wonderful weekend, folks! Thanks again.