You can drill and tap another hole for the second glow plug...I once put 4 more plugs in each cylinder of a racing 289cc twin...Worked well, the 4 others were still working when the center one got too hot and flame was coming out the hole...Melted the hex just like a cutting torch
Turns out they weren't needed, we ran one of the engines for two seasons with changing plugs..Worked so well we were afraid to change...They were McCoy #9s...
Glow plugs come in different heat ranges, the timing will change with the heat range...
Cold plug, retarded compared to a hot plug...