Yes, These types of the promotion are possible. but by default in the Odoo V10 these is not possible. We team Emipro have come with the solution to the problem for managing the promotion like

Buy A @ 0.99p

Buy B @ 0.99p

Buy A & B @ £1.00

We have already contributed the solution the in the form of the the App named Manage Sale Promotions in Odoo for the the general sales operation and Coupon Code in Odoo eCommerce  for the Website sales.

These apps will help customers to create and manage various promotional offers and coupon codes.

Here are some of the highlighting features of these apps:

Select  any of the following Promotion Levels

1) Global Discount - Applies to all products

2) Category Discount - Applies to specific product category or sub-category

3) Product Discount - Applies to specific product and all its variants

4) Variant Discount - Applies to specific product variants

Then, Select any of the following  Promotion Types

1) Fixed Price Discount

Apply flat fixed price discounts by specifying the exact amount.

2) Percentage Discount

Set specific percentage to offer flat discounts.

3) Range based Discount

Set discounts based on Price Range and Quantity Range.

4) Clubbed Promotion

Merge two promotional offers to provide additional discounts.

5) BOGO Offer

Configure Buy One Get One offer.

Other Features

• Set Promotion Duration by specifying start and end dates.

• Set overall promotion budget and specify maximum discount amount per sale.

• Auto Generate Promotion Codes or use your own code.

• Set minimum order amount or quantity of items in an order for promotion to be applicable.

• Set usage restrictions by configuring total number of times a coupon can be used and maximum number of times a customer can use a coupon.

• Configure promotions to make them available for specific customers or customer groups

• Create & Print Barcode/PDF for the Promotion

• Share Promo Code via email with customer.

• Create QR Code for the promotion and on scanning the QR Promotion will be automatically applied to your cart.   

According to your description you can define a set (BoM) for product A+B and sell it at a price you like. A BoM is a Bill of Materials. See this for selling kits: https://www.odoo.com/documentation/user/10.0/manufacturing/overview/bill_of_materials/kit_shipping.html.

Please make sure that you are familiar with the basic ERP concepts such as products, stocks, variants, BoM etc. before you start using Odoo in production. As in any ERP system, if you messed up your setup, you will not be able to correct wrong data once it is entered.

sorry what does BoM mean.. im new to this whole thing and have no clue.

would doing this make it automatically take the discount of £.98p off when scanning through the POS?