As an instructor at Breckenridge I can tell you the staff at Breck would love to help you, too. As justanotherskipro stated a group lesson certainly is more affordable. If the two of you were to do a group lesson together you would have to go with a group that is comfortable for which ever of you is the lower level skier. This is doable during the slower periods of the ski season, but in mid-March the resorts' busiest time of the year (other than X-mas/New Years) it is not really advisable (it can be done though). The higher level skier would not get as much out of the lesson as they would in a lesson of the appopriate level and they can become a distraction for other students in the group as well as the instructor.
Taking a private lesson with both of you together is an option. I teach many privates with people of different ability levels. In this case you will stay on terrain that is properly suited to the lower level skier, but the instructor can give both of you individual attention and instruction. The instructor will be able to give the higher level skier exercises/drills to do on the easier terrain that still have the potential to have a great effect on their skiing.
When I teach privates I always tell the clients they hired me for the day it is up to them to let me know how they want the day to go and I will tell them the options they have. In a case like yours (with the little information I have and not having seen either of you ski) we would start the morning together doing a warm up run giving me a chance to assess both of your's skiing. During this time I would also talk with both of you to see what each of your's motivations are for taking the lesson and what each of your's goals are for the day. If your skiing levels are close together we would all stay together all day. If one of you is quite a bit better than the other I often recommend the lower level skier and myself spend the first two or three hours together while the stronger skier goes off skiing. This allows the lower level skier to get full attention without distractions and without the concerns of being watched & judged while trying to meet the expectations of the stronger skier. After this period of time the stronger skier meets back up and we all ski together with both getting instruction. It is not uncommon the lower level skier will tire out earlier than the stronger skier and want to quit for the day. If this occurs then the stronger skier can stay out for the remaining time going to the terrain they want and receiving all the attention/instruction. Again I leave it up to the client to tell me how they want the day to go when they book a private lesson, the above scenario is just one of the more common ways clients choose they want the day to go.
Another option is to take an "Ultimate 4" lesson (at least at Breck, Keystone should have a similar program). This option is quite a bit less expensive than a private, but more than a group. In an Ultimate 4 the group size is limited to a maximum of four students all of a similar ability level. Think of this as a sort of semi-private lesson. If both of your skiing levels are not too far apart you could both go with the same Ultimate 4 group.
In my opinion, if you have the financial means the private lesson is the way to go. The day is on your schedule taking breaks when & where you want and using the instructors time between the two of you however you want. If the private lessons are financially not possible then I would recommend the Ultimate 4 program. In March the resorts and ski schools are busy, group sizes in group lessons tend to be larger than other times of the season. If you plan on taking a private lesson or an Ultimate 4 at Breckenridge I would encourage you to book it in advance (a minimum of 2 days prior, preferably earlier than that). Breck ski school does have days it sells out and they cut off sales of private, Ultimate 4 and kids lessons.
Enjoy your vacation and ski safe!