You must ask the beneficiaries for their views. If you get a unanimous agreement (and all of the beneficiaries are of full age, capacity and are ascertained), you can freely settle the claim for whatever the beneficiaries decide. The executors should take a neutral role in an IH claim and agree to abide by any order the court makes - however, this will be different if the executors wear two hats - ie they are both execs and both beneficiaries. Remember that this is essentially a claim against the estate and thus the amounts a beneficiary will receive so it is really them who must make the decision on how much to offer.
If you are both beneficiaries and the other wants to make a higher offer, why not suggest that he pays more than you? There is nothing to say that you each have to pay equal amounts.
As a trustee, you have the power under s15 Trustee Act 1925 to compromise any action. However, this must be read with s1(1) of the Trustee Act 2000 which states that the trustees have a duty of care. Although in terms of compromise (unlike investment) there is no duty per se to take advice, a court would be unlikely to criticise executors who acted on the basis of sound advice and with the agreement of the beneficiaries. What you must not do though is settle the claim without the agreement of the beneficiaries since they should also be defendants if a claim were to issue.
However, if agreement cannot be reached between the beneficiaries I suspect the Claimant will issue Part 8 proceedings for an IH claim so you will already be in a situation where the estate is incurring costs.
Divide and rule is often a tactic used when there are beneficiaries who disagree. Given that you seem to be signed up to the idea of making an offer, I would urge you to take advice as to the appropriate level. If at this stage you can't agree, might I suggest you ALL submit to formal mediation in the hope that you can avoid the