It's easier to overdose on a partial agonist.
You can still overdose while on Suboxone, because Suboxone doesn't have to take up 100% of your receptors, unless you're on a huge dose.
If you're on buprenorphine, it's probably easier to overdose, unless you're on a large dose. It's better to only use Suboxone, or full agonists, and not to combine them IMO.
Keep in mind people don't do studies on how "easy" it is to overdose, overdosing is something that should be avoided as a rule of thumb.
How do you figure that it's easier overdose on a partial agonist? Drugs like buprenorphine are very difficult to overdose on because there is barely any CNS or respiratory depression. Tolerance plays a huge role, though. Someone with a low tolerance will have an easier time ODing on Buprenorphine than someone with a tolerance.
As for overdosing on a full agonist while on Suboxone: theoretically, it would be harder to OD because the buprenorphine saturates the receptors and doesn't allow the oxycodone to bind. It's still possible to overdose, but it will take a larger dose than it normally would have.
For example, if you need 40mg of opiate-x to get high, and 60mg to overdose on opiate-x, while on Suboxone, you'll need more like 60mg to get high, while needing more like 80mg to overdose.
Since buprenorphine is stopping the other opiate from binding, there isn't much danger in the two working together to make overdose more likely. Even though you took a full agonist along with the buprenorphine, the buprenorphine is really the only drug that is active, unless you took a massive dose of the agonist to "break through" the buprenorphine.
So, in effect, Suboxone raises the overdose threshold, but be careful because there is no way to know exactly how high the threshold has been raised. It varies from dose to dose and person to person, so if you've recently taken Buprenorphine and want to get high on another opiate, it's best to start with the dose you are comfortable with and work your way up as needed.