Can you practice Buddhadharma and Sikh religion at the same time? Possibly. There are a lot of Buddhist Jews (I was just on retreat with one a couple of days ago). It depends on your understanding of God and how flexible it is. One way or another, Buddhist Jews are bound to view God in a very different way from the average Jew. I don't know much about Sikh theology, but God seems to be less of a person and more of an ultimate reality that one can access through upright behavior and meditation, and that might be compatible. The methods of meditation may be different, though; I really don't know.
A Sikh might have problems performing reverence before Buddharupas, giving offerings and doing prostrations etc., seeing that as idolatry. The fact is that Buddhism has all the external qualities of a religion. At the same time, our Jewish Buddhists obviously don't have a problem with it, so it's up to how you understand both the act and the prohibition against it, to see whether it violates the spirit of the rule.
The last thing is of course that the average Sikh probably finds their religion to be pretty comprehensive and complete without needing to mix in Buddhadharma. I think there is already a bit of Buddhist influence in Sikhi, as there is in most Indian religion, but it has gone in a different direction. And since Sikhs are fine with Buddhists being Buddhists, and Buddhists are fine with Sikhs being Sikhs, some would say it's best for people to dedicate themselves fully to their tradition and try to understand its deepest meaning before they try to mix in other things.
But at the same time I don't think religious purism is terribly realistic, so if people think that bringing in practices from other traditions is helpful and won't cause confusion, then they're welcome to do that. It's not as if all existing traditions didn't arise from that process at some point.
that is a joke btw..