If you take a wire from each OUTER wire of the coilpack connection, through(rectifier) diodes, then join the ends of these two wires at the opposite end to the coilpack this will ensure you get twice as many negative rpm outputs to use with the rev counter you have.
The diodes are directional, ie they have a silver band that must towards the load, in this case the rev counter.
A diode is a one way electrical valve in simple terms.
I would suggest some 3A glass pasivated rectifiers these do cost though, a mind boggling 28 pence EACH!!
Can get these from maplins item number N89CA, on page 805 top right hand corner of page.
If still stuck I have shed loads of these, or if you want a drawing to make mega clear just pm me your email etc.
tabetha