Praise be to Allah.
Islam has brought different kinds of acts of worship, and this is part of the perfection of this religion. There are acts of worship which can be combined with others and other acts of worship which cannot be combined. Fasting on alternate days is one independent type of worship; it is a kind of voluntary fast, and it is the best kind.
Fasting on Mondays and Thursdays is another independent type of worship. It cannot be imagined that it is possible to combine the two on an ongoing basis. The better kind should be given preference, but a person should take his own situation and strength into account, so he should not start something that he will not be able to continue in the case of naafil deeds because the most beloved of deeds to Allaah is that which is continuous, even if it is little. And Allaah is the source of strength.