The idea is to duplicate the page register with a second pair of 670s. That duplicate is used just for back annotation. Maybe not the smartest way to do it, but works.
It has been used in the past (including by myself) because it's a faster circuit. With a single (set of) '670 you need a multiplexer to switch between A0/A1 and A14/A15 (read back registers vs. regular mapper use). That multiplexer + relatively slow '670 gives a delay that can make mappers fail @ 7 MHz etc.
Paralleling '670s takes that multiplexer delay out of the equation. Disadvantage is you have more connections to each Z80 data line (3 per data line vs. just in/out on the 'classic' circuit). These days there are very fast multiplexers, so using those & return to the multiplexer-based circuit may be better option today (if you prefer discrete logic over a small CPLD, that is). In terms of IC count / wiring / cost it often doesn't matter much (but for example '157s are easier to find / cheaper than extra '670s).
Because I needed to mod the BIOS for it to work properly at 60Hz, I also added code to initialize the mapper on boot.
It's definitely a more elegant solution. Most of all because it solves the problem for any mapper inserted, regardless whether that mapper initializes it's own registers on power-up or hw reset.
But it's not a big issue really. If you want to use a mapper on MSX1, just make sure you understand the problem, apply solution and enjoy. 
Because I needed to mod the BIOS for it to work properly at 60Hz, I also added code to initialize the mapper on boot.
It's definitely a more elegant solution. Most of all because it solves the problem for any mapper inserted, regardless whether that mapper initializes it's own registers on power-up or hw reset.
Except when the MSX does not like that we unsolder their chips. Particularly the MSX1 and older MSX2.
Yeah. I hate soldered ROMs 
The common solution these days seems to just cut the legs, and then desolder the remaining leg-parts one by one. I haven't tried it yet, but sounds easier and better than trying to desolder the whole chip at once.
