One of my protegees asked me some time ago to put together some guidelines for opening 6/5 hands when the major is the five bagger and a minor is the longer six bagger--and the hand does not have reverse values.
One thing I need to share up front is that my mentor taught me a valuable lesson in my early years: any 6/5 hand opposite the right opening bid makes slam. Repeat that 100 times before you go to sleep at night.
Reverse values start with meaty 16 counts. One can give some 6/5 hands reverse values even with lower high card point values. Trouble is, those hands have to have playing strength.
For example, AJ654,4,KQ6543,6 is not a reverse strength hand, but something like this might be: AJT98,JT,KQT987,v. So, if you think you have enough playing strength for a reverse, then go ahead and bid out your pattern, minor followed by the major.
If you have a 65 hand without reverse values, then there are to main points to consider: 1) How many suits are in between the two that you have. 2) How strong is one suit relative to the other.
So, for example, if your suits are six clubs and five spades, you have two suits between the blacks, namely diamonds and hearts. So you can handle a bid from your partner at the one level and rebid spades without reversing at the two level. However if your suits are diamonds and hearts, then you're in trouble immediately if it goes 1D 1S--and you don't have the values/playing strength for a reverse. Now you need to evaluate whether switching the order in which you bid the suits may be right.
Another strong indicator of the order in which you should open the suits is the strength of the relative suits. Suppose your hand were: A5432,4,AKJ543,5. Now, one would likely open 1D because you want that to be trump. However, should your hand be: AKJ43, 5,A97543,6 one would likely open 1S.
These bids are hard for us all. Once, vul at imps I held 65,98743,AKQJ54,v. Well, the bridge Gods don't give me solid tricks in my hand very often so I opened 1D. I was most surprised when all the experts chose to pass the hand and "show" the hand later somehow. I don't know how to let on to partner that you passed a six trick hand initially, but apparentl;y the experts do.
So to sum up. There are three things to consider: playing strength, how many suits are between the 6/5 suits, and relative strength of the suits.
And here's the real rub. If you get them all right, you'll be playing much better than the experts.
Hope this is somewhat helpful, please ask if you have further questions.