Assalamu 'Aleykum ! Is valid to perform salat, siting in both heels, instead sitting on left feet? That is because I feel more comfortable in the first way!

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

We have previously clarified in Fatwa 85270 that sitting on the left foot while the right foot is erected, or sitting on one’s posterior are not a condition for the validity of the prayer; rather, in any position the praying person sits, it is sufficient from him; so it is permissible to sit in the manner which you mentioned in the question.

The sitting which you described (i.e. sitting on both heels) is permissible; rather, it is a Sunnah between the two prostrations and it is called Iq'aa'. But this position is not legislated in Tashahhud.

However, we advise you to strive to abide by the Sunnah even if you find some difficulty, as you will soon be accustomed to the Prophetic way of sitting. Indeed, the Prophetic ways of doing actions of the prayer are the best ways for achieving humbleness and attentiveness in prayer.

May Allaah enable us all to abide by the Sunnah.

Allaah Knows best.