Electrical – Using Multiple MLX90614 Sensors on Arduino Board

If they both use I2C then you can attach them both to the same two pins (SDA and SCL)

You select which one you wish to talk to by using it's address for the first byte of each command (which should be given in the documentation along with example communications) For example on page 16 of the datasheet for the BMP085 this is given as 0xEE (write) and 0xEF (read)
It looks like the first sensor has pullup resistors included which means you don't have to add them yourself.

I take it the Arduino Wire library is a software implementation of I2C (as opposed to a dedicated on board peripheral) If this is the case I imagine you can select which pins you want to use in the defines section. So in your case you would select A4 and A5 as SCL and SDA (which ever way round you want them)

The first link I looked at had various helpful links including quickstart guide, datasheet, wiring example, and an example Arduino sketch, so I'd probably start by following those. To keep it simple, I would try and get that sensor talking to the Arduino then add the other one.

I notice that the BMP085 runs from 3.3V, so you will need to use a 3.3V supply and disable the weak pullups to 5V in the Arduino wire library (see comments on product page) You can probably run the other sensor from 3.3V too but you would need to confirm this - it should work fine from 5V with logic high to 3.3V anyway if necessary.

In this answer I showed that the optimal series resistor value is

\$ R_S = \sqrt{R_{MIN} \cdot R_{MAX}} \$

where \$R_{MIN}\$ and \$R_{MAX}\$ are the minimum and maximum resistances of your sensor. The Sparkfun page speaks of between infinity and 300 kΩ. If we assume 10 MΩ as maximum value then your series resistor should be 1.8 MΩ (rounded to nearest E12 value).

These are pretty high values, too high for an ADC, which, like Mathieu says, likes a impedance of less than 10 kΩ. So you'll need to buffer the divider with an unity gain buffer opamp:

Note that a common opamp may have an input bias current as high as 1 µA, and this could distort your reading. A CMOS opamp like the MCP600x has a much lower input bias current, 1 pA typical for the MCP600x, which won't deteriorate your reading.