Is there a way to link my iPad and iPhone together?

1) Perhaps you refer to the Gaia GPS: Mobile Trail Maps App?

https://apps.apple.com/app/gaia-gps-mobile-trail-maps/id1201979492

If so, why not simply install the App on your iPad.

2) Mirroring your iPhone screen to your iPad, this is not supported.

3) Consider the functional differences between WiFi Only and WiFi+Cellular models of iPad. Aside from the obvious Cellular functionality, only WiFi+Cellular models include GNSS/GPS location capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. 

Purchase of a WiFi+Cellular model provides access to precise location services, but does not commit you to using Cellular connections. WiFi Only models of iPad have no Cellular or GNSS/GPS functions at all.

If you have a WiFi Only model iPad, be aware that tethering to an iPhone can only provide internet access via the iPhone's hotspot connection. GNSS/GPS positioning data is not shared between iPhone and iPad. However, you can use a compatible external GNSS/GPS receiver with a WiFi Only iPad.

In summary, your available options are:

• Substitute your WiFi Only iPad for a WiFi+Cellular model; this does not commit you to provisioning Cellular service for your iPad, but does provide the needed GPS functionality for the iPad.
• Add an external GPS receiver to your WiFi Only iPad. These are often used in marine, aviation and vehicular applications - and can be either directly connected via Lightning or via Bluetooth. Their are various third-party manufacturers, of which two are BadElf and Garmin Glo.

What exactly are you looking to accomplish by connecting them?

Are you saying your iPhone does no have cell service, or just that you don't want to use cell service?

There are many features that use Bluetooth between an iPhone and an iPad, direct connection or pairing is neither possible nor necessary. Also, none of them use cellular data service directly. They connect over Wi-Fi locally or the internet.

click here ➜ Use Continuity to connect your Mac, iPhone, iPad, and Apple Watch - Apple Support

are you talking about transferring over your information from the iPhone to the iPad?

If you have an iPad with a cellular data modem, and you want to use it to place cell phone calls, that won't work. It won't work whether you want to place them from the iPad itself, or from an associated iPhone or Mac.

The cellular data modem (in iPads equipped with one) is only for data, and doesn't give you access to cellular voice service.