I personally would have at least two of the same - either M/B or N/M to get a coherent tone (depending on what 'in between' position you like) - I tried a Milk Middle and a Apache Bridge and found the tone to be somewhat wrong - I guess the power of the Milk didn't work with the Apache - I think you could blend Sultan and Apaches in that position though.
In fact if I was going with combo you suggest I would put the Milk in the neck and the Apache in the bridge - my reason is that the bright 50s vibe of the Apache is better capture in the bridge (think Buddy Holly, Dick Dale, Shadows and late) and if you like the Knopfler/Richard Thompson tone in the M/B position your going to want an Apache in the bridge to work with your Sultan.
The woody Alnico V of the MM is going to better put to use in the neck if you like Frusciante, SRV and bluesy Hendrix. I love the Mother's Milk neck position - it capture the Under the Bridge/Zephyr Song type of sound really well and with fuzz does the Hendrix sound on a song like Fire or the intro to Purple Haze beautifully.
As to the Apache bridge being too weak - well it does Shadows easily but I can get a better Hendrix a la the lead break intro to All Along the Watchtower than with the Milks - it really captures that fluid lead sound really well.
As to radius - if you have a vintage style strat (like a Fender 50s/60s classic series) go with vintage (1956) stagger - if you have a modern strat go flat stagger.
If you go with the combo your suggesting - I would go with Stock polarity - I think each pup would keep its character better.
Anyway - sorry if you have an excursion through my musical taste but I hope it helps.
