From the FAQ. This FAQ entry was overturned by the Security Testing + Account Siphon Ruling.

The Runner may choose between triggering an Account Siphon or a Security Testing that has chosen HQ.

Page 11, Column 2, Paragraph(s) 7, FAQ
If Security Testing is used on HQ and the Runner plays an Account Siphon, can he choose which replacement effect to use?
Yes. Both of the cards have the same trigger condition, so the Runner can choose the order that they resolve in (and thus which one is used).
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