Hi Brian, and welcome to Just Answer.
The pens in your Howard Miller desk set Model #613588 might be made by 'Cross', as it says on the website that they take 'Cross' refills.
I have the following suggestions: Contact Howard Miller company and ask if you can order a replacement pen for your desk set and/or Contact Cross Pen company (or go to some stores in your area that sell Howard Miller desk sets and/or cross pens), bring the remaining pen with you, and see if you can find the same one or a similar pen. You said you were willing to replace both pens, if necessary, so if you can find one or a set that fits into the pen holders of your desk set and looks similar to what you have now, this would resolve your problem.
Also, see if you can find any model or style number on the remaining pen in your desk set, and you can search for that on the web or, if you would like me to help you with that, just send me the number and/or the dimensions of the pen (it is ballpoint?) and I'd be glad to research it for you.
Here is Cross pens Customer Service contact information:http://www.cross.com/customer-service.aspx
Howard Miller Contact information:http://www.howardmiller.com/contact_us
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