Why does my mediatek tablet have 2 Sim card slots

I think that tablet might have a hidden slot. First take a close look around all four edges of the tablet. You are looking for a small, long, thin cover slot with a pin hole in it. If there is one, insert the end of a paper clip or a pin to release the spring lock holding it closed. Most New tablets and phones are supplied with a small little 'key' for this.
If there is no pin slot try this. Look at the top edge of the back of the tablet. There should be a plastic trim running all the way across from the camera to the opposite corner. This trim should unclip when you GENTLY lever it loose. There might even be a slight notch or groove in the trim where you lever it free. This should reveal the slots for the sim and the memory cards.

one is enough. Usually one is to give you connectivity to the internet and the other is media storage, OR it is dual SIM for two telco, work and home for instance.

https://www.google.com/search?safe=off&rlz=1C1QJDB_enCA643CA644&sxsrf=ALeKk00Mu6u9JvYZmW9v7f7k1LEeYc7KuQ%3A1615763226302&ei=GpdOYNr_EcTKswW3lbnYAg&q=mediatek+910+specs

your specs

Fast 4G LTE modem with Dual 4G SIM MediaTek G90 series integrates a Cat-12 4G LTE WorldMode modem with 3x CA and 4x4 MIMO and 256QAM. This provides more reliable connectivity performance even in densely populated spaces such as stadiums, busy shopping districts, offices or airports.
Continuing our DSDS (dual SIM) leadership, essential dual 4G VoLTE and 4G data means a seamless user experience from either data connection, while VoLTE and ViLTE services provide the best possible call and live video experience, with faster call setup time noticeably better sound quality.
Advanced cellular features such as IMS (VoLTE\ViLTE\VoWi-Fi), which is now supported by over 175 cellular operators worldwide, and other standards include global eMBMS, HPUE and Band 71 support in North America.

you don't. sim cards are used in phones not tablets

On the left hand side at the top, 3 slots, 2 sim card and 1 sd card slots!

this tablet don't use a micro sim card just a micro SD card (memory card)

First of all, check the device's power is off. Then open the sim card slot and place the sim with gold plating downward and close the slot. You are good to go with your tablet.

i don't think so

If the tablet does not have a SIM slot, then the tablet does not support Mobile connectivity through a SIM card. This will mean that you need to use WIFI to connect to the internet.

http://www.ifixit.com/Guide/iPad+2+GSM+Headphone+Jack+%26+SIM+Slot+Replacement/6159

Follow Step 1 and 2 only.
Note: If you bought the wifi-only iPad then it won't have a sim card slot, if you bought the wifi + cellular version then it should have a sim card slot on the left-hand side towards the top