The ideal is to use identical wire. Barring that, if you only have the two different sizes on hand, surely you an use them to bi-wire. For many years prior to buying entire sets of cables to compare I would swap around different cables on the Mid/Treb and Woofer terminals. It helped to teach me the importance of distinguishing between different cables and their effects.
If you have a good rig and good ears it will be likely that you will immediately notice a difference in the sound if you swap the two cables. You may be very surprised at it. The largest reason for this is the difference in total gauge.
There is no predicting which you will prefer; you may end up liking the sound of improved mid/treble with the larger cable on top, versus the cleaner, fuller bass with the larger cable on the bottom.
Your choice. But, immediately you'll realize that with both sets of terminals having the larger cable you'd have the better mids/highs AND the better bass. You would have just proven to yourself the importance of running the same cable to achieve superior results. :)
If you have a good rig and good ears it will be likely that you will immediately notice a difference in the sound if you swap the two cables. You may be very surprised at it. The largest reason for this is the difference in total gauge.
There is no predicting which you will prefer; you may end up liking the sound of improved mid/treble with the larger cable on top, versus the cleaner, fuller bass with the larger cable on the bottom.
Your choice. But, immediately you'll realize that with both sets of terminals having the larger cable you'd have the better mids/highs AND the better bass. You would have just proven to yourself the importance of running the same cable to achieve superior results. :)