Cluster v/s Regiment was fine in 3050 for mostly L1 tech w/ limited amounts of L2 tech.
By the time of the Jihad, if your talking top end line regiment (Non-RCT) then yes, 2 clusters is going to be a much better figure.
The key is in the support forces.
Is this just 108 mechs (+/- 18 fighters) (1 big Cluster might do okay)
Or is this 132 Mechs, 20+ Fighters, 108 Tanks, & 27+ Platoons of Motorized Infantry.
In the 2nd Options your going to have to use 2 clusters.
Against a full RCT and an assembled planetary militia......pft.... toss a Galaxy at it and hope battle losses aren't too bad.