Can I combine two PRNGs to make use of more seed data?
Why are you afraid of "overseeding" the PRNG? The Java SecureRandom class specifies the setSeed function as:

public void setSeed(byte[] seed)


So just feed the 320 bits (40 bytes) data to the setSeed function and it will supplement the existing seed. Leave the compression into the state to the underlying SecureRandom implementation.



[EDIT1]

Note that the SHA1PRNG implementation simply uses the new seed in a message digest update, and that message digest is SHA-1, hence the 160 bit state.



[EDIT2]

Note that it may depend on the entropy source if the entropy represents 320 bits of true random data. The initial compression of the seed data will make sure that any random entropy from the entropy source is used.

Another reason to combine to PRNG's is when the PRNG algorithm is not fully trusted. In that case it is much better to switch to another PRNG. If one is vulnerable the other one will be vulnerable as well.

It is better to re-seed the PRNG more often if there is entropy to spare.