I'm not sure I follow this, but I'll describe what it does and what the alternative might be.
When you overdub in reverse (or speed) the effect is applied to the overdub as it is being recorded. So record an initial layer normally, then put it in reverse, overdub something. As the two layers play, the first will sound reversed and the second will sound normal. When you turn reverse off, the first will sound normal and the second will sound reversed. Call this N1+R2
You can also make the initial recording in reverse. Press Reverse before you press Record to "arm" it. Then record something. The loop will sound normal since it was recorded and is playing in reverse. Now keep it in reverse an Overdub. While it is in reverse, both layers sound normal. When you turn off reverse, both layers sound reversed. Call this R1+R2
What you can't do is have the reverse be effecting layer 1 but NOT also be effecting the overdub. Record R1, overdub N2, turn off reverse and have N1+N2. That doesn't really fit with the "tape model" of recording. The direction the tape is moving effects both what you hear and how you record. If that is what you want, it's a rather large change to the way the old code handles recording. Before I invest the time on that, I would probably rather make it easier to do using multiple tracks and bouncing. Record track 1 and leave it in reverse. Record track 2 normally, bouncing track 1 into it. In effect track 1 becomes the "first layer" of track 2 and you can record over that any way you want. The result is a single layer containing R1+N2.