I haven't seen any statistics, but people who study the church have noticed that a growing number of Christians in America are involved with two churches. I know some people who primarily serve in one church, and are fed in another church. Other people split their time between two churches because they like different programs at each church, like worship in one church and the children's program in another.
The first thing we should consider is what we mean by church and belonging to. In the first century, Christians already used the word church in several ways: individual house churches (Rom 16:1, 5); all the churches in one town (Acts 11:22, Rev 2:1), and all Christians everywhere (Acts 9:31, Col 1:18). The most important meaning is the last one: all Christians are "baptized by one Spirit into one body" (1 Cor 12:13). So all true Christians, regardless of location or denomination, belong to the one true church. In the first century, Christians were "members" of one house church, but sometimes several house churches met together, and sometimes people (especially leaders) would visit other house churches.
So there is nothing wrong in principle with attending more than one church; in fact, it may be evidence of unity between Christians. But there is a second question: what does it mean to belong to a church? If we walk through the pages of the New Testament, we see a whole list of "one anothers" that should define our churches: love one another, value one another in humility, serve one another, build up one another, forgive one another, be united with one another, and show affection to one another (see here for 60 passages describing how Christians should treat one another). And that's only a start - we could add a whole list of other things that we should be doing as members of our churches.
With that in mind, we need to ask a question about motive: am I attending so that I can be part of the "one anothers" at both churches? Or am I only church two-timing - trying to get what I want from each church? It's possible to bless and be blessed by two churches - or it's possible to just take from each church and give nothing. Whether we attend one or two churches, we need to fully participate, giving and receiving.
If your involvement at two churches blesses other Christians and blesses you, then it may be a good thing. I have a small group Bible study that has people from four churches, and it helps build up all four churches. I attend other churches when they invite me to preach. But if you are just a consumer at both churches, then you should work on engaging in the "one anothers" at just one church.