Ok, well I'm trying this horse over the weekend, and the lady gave us his bit and bridle, but she was riding in a different bit on another horse, and she didn't have a converter strap for the rubber mouth phelm that he REALLY likes. So how can I ride in his bit without a converter strap? Or can I do something to quick fix it?
You may be able to use a flash noseband? Not sure... Or a monkey grip/grab strap. Twine shouldn't break, as long as you tie it on well.
The OP said she wanted ideas to replace a converter, which allows the bit to be used with one set of reins. If the horse is used to it, I personally would rather find an alternative than ride with two, one because that is what the horse is used to, and two because I have never used double reins in my life. The OP may be in the same position. Doesn't make you less of a rider.
I WOULDN'T advise riding with double reins on an unfamiliar horse if you have never used them before.
But, as far as I know, with double reins you hold the snaffle rein as you normally would, and the curb rein in between your fingers one step up from the snaffle rein. Lol if that makes sense. Like this: