Is it permissible to combine the travel allowance paid by the employer with the travel expenses covered by the host entity?

If the labor law allows you to receive transportation allowance from two entities, then there is no harm in that. However, if you are only entitled to receive it from one entity, then you are not permitted to take these expenses except from one entity of your choosing. You must fulfill the conditions you committed to when contracting, based on the Almighty's saying: "O you who have believed, fulfill [your] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions."