will having two earthing points cause a problem?

It is my understanding (it came up recently) that you can bond the supply
earth with your local earth rod. After all, main equipotential bonding also
connects your supply earth to local earth. The local earth rod is essential
for generator supplies, as you can't rely on the supplier earth in the case
of a power cut. This is because the power cut is possibly a cable break,
which is as likely to affect the earthed armour/combined neutral earth
conductor as the phase conductor.

You must ensure that the entire system, or the generator circuit is capable
of running as a TT system. This requires the earth rod, as mention before
and a suitable RCD in the loop somewhere. A 100mA Type S is indicated. This
can go just after the generator and before the transfer switch, in which
case you have TN earthing on mains and TT on generator. Alternatively, the
RCD can go after transfer switch and the supplier's earth disconnected. This
will give you TT at all times. The instructions on the transfer switch
should be followed, however.

Christian.