I've done this a few ways in the past.
One way is just to build the experiment as a one-concept CBC with a standard None alternative. Then simply type the constant attribute levels into the None alternative. Unfortuantely in your case, it is Alternative 2 that will be constant in this arrangement.
The other way is a bit more complicated. Create the experiment with two alternatives. Then export the design and manually code the first alternative always to have your fixed levels. Then import the design back into the Lighthouse or SSI Web software
Better still would be to build this as a once concept CBC with a standard none. Then export the design and modify it in Excel: make the one concept be concept 2 in each choice set and add a concept to be the fixed concept 1. Again, import into the Lighthouse or SSI Web software and you're ready to go. This may give you a slightly better design than the second approach above.
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