Some halachic authorities believe that the scholar loses part of his reward in the World to Come due to the agreement (the Netziv in "Meshiv Davar" 3:14 and the opinion of Agudat Yisrael). However, most authorities maintain that the scholar's reward is not affected at all (Orach Chaim, Shemot, Chida in B'rosh David, Hafla'ah in the introduction to Ketubot, Chazon Ish, and others).
According to most halachic authorities, it is permissible to make an agreement with multiple donors, with each sharing in the reward in the World to Come only for the Torah they helped study.
The primary goal of these agreements is to increase the light of Torah in our world and to fulfill the will of our Creator. 
The focus is not on receiving reward in this world or the next, but on doing the will of God out of love. In the end, the reward will come.
There is no need for a written document; an oral agreement is sufficient (Igrot Moshe, Vol. 4, p. 67).