You can't call it a good review if you've only looked at two sources, and both have the same point of view. Some of the biggest internal threats to the validity of a study are personal bias and not having a diverse enough sample. With a literature review, your sample is your list of sources. Your review should be comprehensive, so you should include all major research on your topic. Make sure that the research comes from authors from a variety of universities, countries and ethnic backgrounds, if relevant. If your sample is not large or diverse, discuss how this affects the body of research.
If your literature review summarizes scientific studies, the methods used in those studies will be important. For example, the studies should include placebo groups so participants don't know what treatment they are receiving. The studies should also include measures to ensure anonymity of some personal details. Otherwise, the behavior of the participants or the researchers could have influenced the results of the study. These are all internal threats to the source studies. While you can't control them, you can make note of these variables in your own literature review to reduce the internal threats to your work.
The expertise of the authors included in your literature review is essential to its internal validity. If the authors are inexperienced, have produced questionable results in the past or have biases that may influence their research, this can cast doubt on their research. While you can't leave out major research in your review, you should make a note of any questions you have about the authors to protect the integrity of your review. In some cases, you can throw out a study from your literature review completely, but if it is well-known in the field or has made some interesting claims, you need to explain why you omitted it.
After research has been published, it may have become discredited because the author was found to have plagiarized, the results were proven inaccurate, or the methods were shown to be questionable. Changes in technology or other advancements may have also shown the study to be limited or inaccurate. If you rely solely on the original source material, you will never learn about these external threats to the validity of your study. It is important that you look for additional research or responses to the research you include in your review.