DT3.
Does having the spark plug in either hole in the head make any difference to the running of the bike as there is no mention of this in the owners manual.

Makes no difference. One day i'll devise a way of running two plugs & see if it improves performance.

I'd recommend running the plug in the correct hole...  not that you'd likely feel a difference.

Second hole was there as a convenience to install a compression release... a popular mod in the early 70's that quickly fell out of favor.

If you decide to install a "spare" spark plug in the extra hole, make sure you still carry a spare with you.  The "spare" plug in the head is exposed to the same conditions and will often foul if the main plug fouls...  not a reliable spare. 

Having a second spark plug also reduces the compression slightly compared to a solid blanking plug...  again, you'd proably never feel a difference...

Quoteing doesn't work on my computer--don't know why, just says error & message box empty it says.--Anyway, copy & paste it is.

Mauser says. I ran an extra coil and lead and altered the timing to give a spark on exhaust it now delivers twice the HP.....sounds like you purchased the low power model same as me....?

Are you saying the mag puts out enough grunt to power two separate coils. I'm sitting up & taking notice now lol. Not sure what you mean by "altered timing to give spark on exhaust". Sorry if i'm mucking up the origial post.

I think that Mauser's tongue was between his cheeks - sorry, I mean in his cheek - when he made his post!

Ok ya had me half sucked in haha but thought what the hell were ya talking about regarding timing.
I did run a twin lead harley coil once & twin plugs but needed the gap almost closed to jump two sparks. It ran but was short lived. Has go me thinking though as i can get more volts from a lighting coil so might try it using two separate plug coils powered by the lighting coil & experiment with the different voltages from charging options from that coil. I had a farm 100 running well using the lighting coil on lowest output wire. Bike had a crook ignition source coil.
But my current idea--i have a 360 running cdi source coil & no pulser coil but use a Suzy RG50 secondary coil & cdi box combined. Just one wire from the source coil to one connection into the coil/box. Runs well. So i'm thinking if i had a stator plate to take two souce coils exactly 180 apart then use two of the coil/box set ups then it should run two plugs perfectly. The cdi sparks twice a lap so should both fire at the set timing & also fire at bit before bdc. Can strobe them as i had at work on AG100 cdi farm bikes & found it happening twice a lap. Also with my cockup system i followed the white mark & found it at bottom & top--so now i need a base plate--i'll find one.--he said hopefully.

My DT400 only has one spark plug hole in the head but it did come with this cool vintage looking dual spark plug holder. Threaded for two spare plugs top and bottom.
 

Very popular in the 70’s Missing the wrench. I still have the wrench but not the holder