This isn't possible with <openended>. While it is possible to "disable" the next button (by either moving it off-screen or by making any response invalid per /isvalidresponse), you cannot add another button, nor does the <openended> element allow for <text> elements (as buttons) to serve as responses. The options are:
#1: The two-trial approach you already outlined ("So far the only way I can see to work around by using openended and the next button and to then confront the participant in another trial with the two options.").
#2: A <surveypage> with a <textbox> and an additional question (e.g. <radiobuttons> or <dropdown> presenting the two options "Help me with this task" and "Confirm". You can then /branch to a different page based on the response to that 2nd question.
#3: Building a sort of "custom" <openended>-like interface using a number of <trial> elements. This option is complicated and not necessarily workable. Whether it is or not depends on the type / complexity of input you need to accept (you say "a number", which can mean a lot of different things, from a single-digit integer to large real numbers) and what you then want to do based on that input (if anything).
Option #2 might be a suitable compromise.