Is it permissible for two people to use a paid design program with a single subscription, for two different projects, knowing that the company allows its download on more than one device provided they are not run simultaneously?

There is no harm in using the program on two different computers alternately, as long as the company permits it and stipulates that it not be operated on both devices simultaneously. This applies whether working on one project or two different projects, as the condition of alternation is met in both cases.