How would I get a divorce if I’ve been separated 2 years and don’t know where my spouse is?

Legal action cannot be taken against a person without giving them an opportunity to appear and explain their side in court. Consequentlyly, notice must be given to them. In a divorce situation where there is a missing spouse they still must be informed of the divorce action before it can proceed. This is what is accomplished by what is called "divorce by publication".
The filing spouse a/k/a the "petitioner" must make a diligent effort to find their absent spouse a/k/a the "respondent". They will have to present proof in court that they made a genuine effort to find the other spouse. Typically they'll have to show that they checked with family/friends, DMV, voting records, DMV, etc. 
At that point, the petitioner will be allowed to serve the repondent by publishing notice of the divorce in a newspaper since they can't use personal service. You will instructed as to which paper should be used by the judge it is usually in the one that is in the area of the absent spouse's last known location. 
In most jurisdictions, the respondent will have around 30-60 days to file an answer. If they fail to respond within that time, then the petitioner can file a request to enter a default divorce. As a general rule, it is granted upon the terms requested by the petitioner although the respondent is given a certain period in which they can appeal the divorce and/or the its terms.
Here is a link to a site that will help get you started
http//texaslawhelp.org/resource/divorce-without-children-in-texas?ref=RokAC