Glyphs must be overlapping for combination to occur (or, if a creature is affected by the Glyph of Paralysis, the Glyph of Repulsion has to touch the paralyzed creature).
No physical/mental resistance check is performed. A Spell resistance check is performed only when the order of casting is Paralysis and then Repulsion. Most boss-level creatures shrug off Glyphs of Paralysis, Glyphs of Repulsion, Paralysis or Mass Paralysis spells (due to high physical/mental resistances) but very few have any respectable magic resistance or use any anti-magic spells, so they will most likely be trapped by the Paralysis Explosion.
The Paralysis Explosion is centered at the point where the second glyph is cast. It might be difficult to select the point at which to cast the second glyph so as to paralyze your enemies while leaving your party unscathed, for the cursor will only show the AoE for the second glyph, not for the resulting Paralysis Explosion. However, if someone in your party can cast the Fireball spell, which has the same radius of effect as the Paralysis Explosion (that is, 7.5 m), it is easy to use the Paralysis Explosion precisely at the point where you need it. To do so: A) cast the first glyph at the approximate area where you wish the Paralysis Explosion to be centered; B) pause the game and select the character who can cast the Fireball spell; C) using the Fireball cursor, select the exact point where you wish the Paralysis Explosion to be centered, and do not move the cursor; D) using the F1-F4 keys, select the character who will cast the second glyph; E) select the second glyph using the hotkey and cast it at the point to which you moved the cursor when targeting the Fireball at step "C". It is done – the Paralysis Explosion will occur exactly where you need it. (Needless to say, you need to put the second glyph to the quickbar slot which has a hotkey tied to it prior to following these steps.)