This magazine is issued only once every two weeks.
This means you get the delivery or the update of the magazine only one time in the span of two weeks. Magazines and special edition newspapers usually follow a basic trend of following a schedule. So if you get a magazine today, the next one will be issued on (or before in some cases) November 13th.
This magazine is issued twice a week.
This means you get the delivery or the update of the magazine two times in a span of one week. Similar to the previous case, the dates may be fixed, like Monday and Thursday or maybe Saturday and Sunday, likewise.
And yes, biweekly can be used to mean either every two weeks or twice a week. Note: Biweekly, as the meaning says can be used to mean either instance, which will certainly lead to ambiguity. Better avoid it.
There is a special word for twice a week occurrence: Semiweekly Occurring twice a week. (North American usage)
Again, there is another noteworthy word for once every two weeks or simply, every two weeks: Fortnightly Happening or produced every two weeks. (British usage) (credits to @rjpond)
Some useful blogs you can refer to:
- Bi vs. Semi
- Biweekly vs. Fortnightly vs. Semi-weekly