The text of the Heimdall 2.0 (https://netrunnerdb.com/en/card/03015) says: "The Runner can spend 2 clicks to break up to 2 subroutines on Heimdall 2.0".
Is this a one-time use perk or something the runner can use several times, for example, spending 2 clicks twice (as two different actions)?
The runner can use this option multiple times, provided he/she has enough clicks to spend, same as with 1.0 model bioroids. Main difference from 1.0 is that runner cannot spend just one click to break exactly one sub with 2.0 bioroids.
You can spend exactly two clicks to break up to two of Heimdall 2.0's subroutines. If you have fewer than two unspent clicks when you encounter him, you cannot spend clicks to break subroutines and will have to break them some other way, or let the subroutines fire. In theory, you could spend two more clicks to break the one remaining subroutine, if you had gained an extra click and had four left at the start of the run.
N.B.: The 1.0 line of bioroid ice lets you spend clicks to break subroutines at a rate of 1:1.
