Persian Sparabara Units

Err – I don’t understand. SPQR has rules for Spara, so far so good, and you can equip Levies with them, again good. But the Sparabara troops were formed into units where only the front rank has the spara and spear combo and the rest have bows – technically there is some discussion as to whether the back rank has spear & bow, or possibly the front rank may have spear, bow and spara. This isn’t possible in SPQR as everyone in the unit has to be equipped the same. Any suggestions?