没这种美事儿. 如果你在某几年报税时已经指定你的其中一个房子为自住, 这个自住放卖了后, 你可以指定你的另外一个房子为自住, 但是后一个自住房的自住期不能包括前一个房的自住期. 也就是说, 你不能同时拥有两个自住房.
到底"某几年",须看看下面那个文件来确定。咱先学好文件,领会精神,再具体操作。税务大人,咱惹不起啊!
我不急着卖,我永远只有一个principal residence ,成么?须住多少年,那房子才算我的principal residence,我多住一年再卖,如何?

This bulletin discusses the principal residence exemption, which can eliminate or reduce (for income tax purposes) a capital gain on the disposition of a taxpayer's principal residence.
In order for a property to qualify for designation as the taxpayer's principal residence, he or she must own the property. Joint ownership with another person qualifies for this purpose.
The housing unit representing the taxpayer's principal residence generally must be inhabited by the taxpayer or by his or her spouse or common-law partner, former spouse or common-law partner, or child.
A taxpayer can designate only one property as his or her principal residence for a particular taxation year. Furthermore, for a taxation year that is after the 1981 year, only one property per family unit can be designated as a principal residence.
If the land on which the housing unit is situated is not in excess of one-half hectare, it usually qualifies as part of the taxpayer's principal residence. Land in excess of one-half hectare may also qualify, but only to the extent that it is established to be necessary for the use and enjoyment of the housing unit as a residence.
If the taxpayer's principal residence is located on his or her farm, the taxpayer has a choice of two methods for determining what portion of any gain on a disposition of the farm can be eliminated by the principal residence exemption.
A complete or partial change in the use of a property from a principal residence to income-producing, or vice-versa, results in a deemed disposition of the property by the taxpayer at fair market value. The taxpayer may be able to elect that the deemed disposition on a complete change in use does not apply. A property covered by such an election may qualify as the taxpayer's principal residence for up to four years, or possibly longer in the case of a work relocation.
It is also possible for a personal trust to claim the principal residence exemption on the disposition of a property. Modifications to the normal principal residence exemption rules exist for this purpose.
The above topics are discussed more fully below, as well as other topics relating to the principal residence exemption.
The appendices to the bulletin contain illustrations of some of the rules discussed in the bulletin.
http://www.cra-arc.gc.ca/E/pub/tp/it120r6/it120r6-e.html