I HAVE A CLIENT WHICH IS TWO PERSON LLC. IS IT PERMISSABLE TO MAKE A CONTRIBUTION TO ONE
PARTNER WHILE THE OTHER PARTNER OPTS OUT OF THE PLAN? I KNOW THIS IS PERMISSABLE IN A
PROFIT SHARING PLAN.

A condition of having a SEP Plan is that IRAs be established for ALL eligible employees and ALL eligible employees generally are to receive a uniform contribution as a % of compensation. If you do not have this you do not have a SEP. There is no option for "opting" out other than having the contribution made and the participant can elect to withdraw the amount from their IRA, taxes, penalties, etc.

It sounds like you need to adopt a profit sharing plan and make one partner ineligible if one does not wish to be a part of the plan.

Have the partners both met the maximum eligibility (3 years of service and age 21)? If not, perhaps you could establish eligibility requirements which exclude one and not the other?