Can a lead exist in two different partitions?

We want to use a lead partition to do some general outreach to companies we have partnered with (~70 contacts), but we want to keep that communcation separate from anything happening in our default partition. However, Marketo is not allowing me to upload the complete list of these contacts to our partition for the following reason:

existing lead (ID 1173109) is in inaccessible partition ID 1003

or

existing lead (ID 1017286) is in inaccessible partition ID 1

Any ideas on why this is happening? I was under the impression that lead partitions do not communicate with eachother and this should be possible?
 

Thanks!