Malbim asks this question in his commentary to 31:45. He suggests that this was a two-part pact between Lavan and Ya'akov. They were treat each other properly both publicly and privately. The former can be supervised by other people, while the latter can only be overseen by God. The two monuments correspond to these two elements of the pact. The גל is supposed to be a large visible structure (note that it was composed of multiple stones, took multiple people to assemble, and was large enough for multiple people to sit and eat on) to remind witnesses to the pact of that pact, so that they keep the participants honest in their public dealings. The מצבה is to metaphorically "remind God" to supervise their private behaviour. It need not be large since it is only metaphorical. Furthermore, Malbim suggests that מצבות in particular were of religious significance at that time, so it was appropriate to serve as a "reminder to God." Regarding this significance, note Genesis (28:22 and 35:14) and Leviticus (26:1).
Interestingly, Malbim's explanation was largely anticipated by the Midrash B'Reshit Rabbati (Parashat Vayetse 31:48-50: pg. 144) which states:
א"ל לבן ב' דברים יש כאן מצבה והגל...כשם שהם ב' דברים כך אני מבקש שיהיו משמשין ב' עדויות. הגל הזה יהיה עד ביני וביניך שאתה לא תרע לי ולא לבני, ואני לא ארע לך ולא לבניך, וגם בניך ובני יעמדו בברית הזה. והמצבה תקרא את שמה מצפה, למה כן, יצף ה' ביני וביניך, שלא תרע לבנותי ולבניהן, הה"ד והמצפה אשר אמר וגו' אם תענה.
Lavan said to [Jacob] there are two things here: a מצבה and a גל. Just as they are two things, I want them to correspond to two testimonies. The גל corresponds to you not harming me and my sons, and to me not harming you and your children, and so too our descendants. The מצבה is called מצפה (looking) since God should watch you and make sure you do not harm my daughters and their descendants. As Scripture says (31:49-50): And the מצפה...That you should not harass daughters.
Shadal (commentary to 31:46) suggests that the מצבה was more of a Hebrew custom (cf. Genesis 28:18 and Joshua 24), while the Aramaean custom was to erect a גל. Therefore, Ya'akov built the former while Lavan's brothers built the latter (this depends on whom לאחיו in verse 46 refers to). He notes that in 31:52, Lavan describes both monuments, but indicates that it is the גל that he will not violate אם אני לא אעבור אליך את הגל הזה). Assuming that he is right that Hebres erected single stone monuments, while Arameans erected many, he suggests that perhaps both kinds of monuments corresponded to their deities; a single stone to the Hebrews' single deity, and multiple stones to the Arameans many deities.
It should be noted that although Rabbenu Avraham ben HaRambam (there) considers the reading of the גל and מצבה as two structures, he also suggests that they could be a single structure, or parts of a single structure.