Unless the leasing organization provides for a 10% of pay (Money Purchase Plan) type of contribution, you cannot exclude these employees from your plan. The plan you have described is not comparable to what is required.
Since you and the other employer and unrelated business entities, you cannot "offset" contributions made to that plan from your SEP.
Any chance you are older (eh, more mature.... ) than the bulk of the other employees?
If so, you may be able to benefit from a new-comparability / cross-tested profit sharing plan.
If this is true, you should seek out a qualified Third Party Administrator in your area. Be sure to ask if they handle cross-tested plans, many don't have the capacity for this type of plan.
Note- many CPA's are still not aware of their existence - so exercise caution with whatever advice a CPA may give. (I do not mean any offense by this statement. It has just been my experience that many CPA's steer clients toward SEP's, Simple's, etc. regardless of what is really best - they just don't know any better..............)